This relates generally to the growth of lower defect materials on highly lattice-mismatched substrates.
It is desirable to grow disparate materials on top of one another. Some materials have such inconsistent crystallographic structures that, if one material is grown on the other, a high number of defects may result. The possible defects may include threading dislocations, interfacial or horizontal dislocations, dislocation loops, voids, nanopipes, small angle grain boundaries, and pits.
For example, in order to grow gallium nitride on sapphire with low defect density, various approaches have been proposed. Gallium nitride is of particular interest because it has a relatively high melting point, carrier mobility, electrical breakdown field, and a bandgap for light in the blue-ultraviolet regime, making gallium nitride desirable in high power optoelectronic applications.
In one approach, known as epitaxial lateral overgrowth, the substrate surface is patterned with a dielectric material, such as silicon dioxide, with periodic openings. Then the gallium nitride is selectively grown in these openings. Once the gallium nitride grows to a thickness of the mask layer, it begins to grow laterally, coalescing into a continuous film. While this works, the number of defects that are formed may be undesirable.
Generally, it is difficult to grow or deposit one material on another material without introducing a significant number of crystal defects when the two materials have different symmetry and/or lattice constants. However, it may nonetheless be desirable to do so.
Various materials may be crystallographically inconsistent with one another. Such materials may be called lattice-mismatched material systems or lattice-mismatched heterosystems. Examples of such lattice-mismatched heterosystems include gallium nitride on sapphire or gallium nitride on Al2O3. In such cases, a relatively large mismatch strain occurs, for example, greater than about 15 percent, if one of these materials were grown directly on the other. Direct epitaxial growth of gallium nitride film on sapphire has extremely high dislocation densities, largely interfacial and threading dislocations, which may significantly deteriorate the photonic and microelectronic device quality and performance. Thus, significant effort has been made to reduce the dislocation density of epitaxial layers on gallium nitride.
Among the possible applications of such lattice-mismatched heterosystems, single crystalline gallium nitride devices show promise for realizing photonic and biological nanodevices such as green light emitting diodes, shortwave ultraviolet nanolayers, and nanofluidic biochemical sensors.
Of course, the problem with these devices is that they necessitate the formation of highly lattice-mismatched heterosystems. The systems are lattice-mismatched because there is a discontinuity in either or both of the symmetry and lattice constant between the two materials. Generally, symmetry and similar lattice constants may be needed in order to successfully epitaxially deposit one material on another. In the absence of such matching of lattice constants and symmetry, techniques are needed that result in lower defects.